The objective of the proposed research is to investigate light- dependent ionic channels in the plasma membrane of photoreceptors, and intracellular calcium changes that may contribute to the regulation of their activity. A large fraction of the experiments will make use of invertebrate photoreceptors, as they constitute an excellent model system for studying molecular aspects of visual excitation under physiological conditions. Experiments are designed to record both macroscopic and single channel currents in various classes of enzymatically isolated cells (ciliary and rhabdomeric), to characterize multiple light- dependent conductances. The effect of light adaptation, local stimulation and membrane voltage on single channel activity will be investigated, and direct action by cyclic GMP and calcium ions will be assessed on different channel types. In addition, the changes in intracellular calcium that occur in relation to visual excitation -- either directly or through specialized voltage- sensitive calcium channels -- will be monitored by means of recently developed optical techniques. The time course and spatial distribution of calcium changes under a variety of conditions will provide information on underlying mechanisms and their involvement in modulating the visual excitation process. Optical calcium measurements will be performed also on amphibian solitary photoreceptor cells.